Illustrated herein in embodiments are toner processes, and more specifically, emulsion aggregation and coalescence processes. More specifically, disclosed in embodiments are methods for the preparation of toner compositions that include a wax component by a chemical process, such as emulsion aggregation. In the process, polyester toner binder resin (for example, a crystalline polyester or a mixture of crystalline polyesters or an amorphous polyester or a mixture of amorphous polyesters), are solvent flashed together with wax to emulsify the resin and wax for incorporation into the toner. The resin and wax combination is then aggregated with the optional addition of a colorant dispersion and/or additional wax emulsion, shearing and adding an aqueous solution of acid until the pH of the mixture is from about 4.0 to about 5.5, heating to a temperature of from about 30° C. to below the glass transition temperature (Tg) of said resin, wherein the aggregate grows to a size of from about 3 to about 20 microns, raising the pH of the mixture to a range of about 7 to 9, heating the mixture to above the resin Tg to coalesce said aggregate, and optionally decreasing the pH to a range of 6.0 to 6.8 to provide toner size particles.
Waxes are generally added to toner compositions in order to aid in toner release from the fuser roll during fusing. Wax also helps release of the fused image document from the fuser roll, that is, to prevent the fused image document from curling around the fuser roll. Wax is especially useful for this purpose in oil-less fuser designs, where oil such as silicone oil is not present to perform these functions. Further, waxes in toner formulations aid in the prevention of document offset, where it is undesirable for fused images on documents in contact over a prolonged period of time or at elevated temperatures to be transferred from one document to another (toner-to-toner and toner-to-paper). In fuser designs that utilize stripper fingers to aid the removal of the fused image document from the fuser roll, waxes are also generally added to the toner formulations in order to reduce the occurrence of stripper finger marks on the fused images (scratch marks, changes in image gloss, and the like).
Carnauba waxes, such as RC-160 (Toa Kasei Co., Ltd., Japan), and fatty acid amide waxes, such as KEMAMIDE S-180 stearyl stearamide wax (Crompton-Witco, USA) are particularly useful in polyester toner designs such as for application where high gloss is a requirement. In classical emulsion aggregation toner processes, wax has been added to the toner formulation in the form of an aqueous emulsion or dispersion of solid wax in water where the solid wax particle size is, for example, in the range from about 100 to about 500 nanometers. The solid wax particles in the emulsions need to be stabilized with an emulsifier such as for example a surfactant. Processes for producing wax emulsions wherein surfactants are used as stabilizers are well known. For example, RC-160 carnauba wax can be emulsified by mixing it into deionized water containing about 2.5 parts per hundred (surfactant to wax ratio) anionic surfactant, heating the mixture to about 105° C. in a closed reactor, homogenizing the mixture for about 45 minutes at 8,000 pounds per square inch in a high pressure piston homogenizer, and then cooling the product to room temperature.
To achieve desired low temperature fusing performance in toner formulations, it has been found advantageous to utilize a blend of crystalline and amorphous resins such as polyester resins in the toner formulations. Crystalline resins alone in toners generally provide excellent low temperature fusing and high gloss performance, but generally provide poor fusing latitude. Amorphous resins alone generally provide excellent release performance, but generally their low temperature fusing performance is limited by blocking and document offset requirements. By mixing both crystalline and amorphous resins, one can achieve both low temperature fusing performance and wide fusing latitude.
In some toner formulations, sulfonated polyesters have been used in the resin designs wherein the sulfonated groups enhanced the emulsifiability of the resin and promoted the aggregation and coalescence performance in toner preparation. However, in some cases it has been demonstrated that the presence of the sulfonated groups is detrimental to blocking performance and relative humidity (RH) sensitivity at high temperature and high humidity (about 80° F. and about 80 to about 85 percent relative humidity).
A known process for emulsifying nonsulfonated polyester resins is by solvent flashing wherein the resin is dissolved in an organic solvent such as for example ethyl acetate at an elevated temperature but below the boiling point of said solvent such as for example 65° C. The resulting solution is mixed into water containing an anionic surfactant such as Taycapower BN2060 (Tayca Corp., Japan), mixed with a homogenizer and then heated to a further elevated temperature above the boiling point of said solvent such as for example 80° C. to flash off the solvent and then cooled to room temperature.
Robust and repeatable aggregation and coalescence performance has proved to be a challenge in working with high acid number (such as greater than about 12 milligrams KOH per gram) polyester resins containing substantially no sulfonated groups and stabilized with anionic surfactants. Improved success, however, has been demonstrated in processes containing resin and pigment by significantly reducing the amount of surfactant in the ingredients and utilizing pH adjustment or aluminum sulfate for coagulation resulting in excellent relative humidity and high temperature/high humidity charging performance. To enable low-surfactant aggregation and coalescence toner processes, it has been possible to produce crystalline and amorphous linear polyester emulsions containing substantially no surfactant by solvent flashing, wherein the resins are stabilized with bases such as for example sodium bicarbonate or ammonium hydroxide.
The incorporation of wax emulsion stabilized with anionic surfactants into the emulsion and aggregation toner processes containing reduced levels of surfactants has proved to be further challenging. Still further, it has not been able to produce surfactantless wax emulsions. More specifically, attempts to solvent flash waxes such as carnauba wax or stearyl stearamide wax utilizing sodium bicarbonate as the stabilizer wherein the wax is dissolved in ethyl acetate at 65° C. have not been successful. It has proved to be advantageous to incorporate wax into the toner processes wherein the polyester resin and wax are emulsified together utilizing bases such as for example sodium bicarbonate or ammonium hydroxide as the stabilizer such as by solvent flashing and with reduced or substantially no surfactant.
Reducing the addition of surfactants into the emulsion aggregation process toner has a further advantage of reducing the need to remove the surfactants in toner washing processes such as to enable satisfactory toner charging and development performance.
Yet further, emulsifying wax to submicron sizes can be a costly process. Alternatives to wax emulsification such as by combining it with resin emulsification are thus even further desired.
The processes of the disclosure, in embodiments, provide a means for toner compositions to be made faster and at lower cost utilizing nonsulfonated polyester resins, by allowing the resin and wax to be emulsified together by solvent flashing with reduced or substantially no surfactants such as to enable substantially complete incorporation of said resin and wax into the toner.